content model of reltable for bookmap

I just noticed that you cannot put the various bookmap
specializations of topicref (chapter, preface, etc.) into
a reltable.  This seems like a potential oversight.

I would think that a common way to populate a reltable
would be to copy elements from other parts of the map
into the reltable, but the way things are setup now
this isn't possible.

Does it make sense to add the bookmap specializations
to the content model of reltable for the bookmap doctype?

paul